1. Field of the Invention
The present invention relates to an ISDN (Integrated Services Digital Network) private branch exchange (PBX), and more particularly to a technique for realizing a fault tolerance capable of sustaining a subscriber service in progress even when an ISDN PBX has unexpected troubles.
2. Description of the Related Art
An ISDN PBX has a data communication function as well as the existing voice communication function. Recently with the development of text and image communication, the ISDN PBX also includes a multimedia function which can process voice, data, text, and images. Further, with the diversification of the services in such an ISDN PBX, it becomes very important to secure a reliability of the exchange system.
The reliability of the exchange system refers to a capability of performing a function without trouble. In order to improve the system reliability, there have been proposed a few approaches: one is a fault prevention, and another is a fault tolerance. The existing fault prevention is to realize the system prepared for the worst by using high reliability elements, to prevent all expected errors in advance. However, the fault tolerance realizes the system by using standard elements. Thus, although there may be a possibility of errors, the fault tolerance dualizes the system by using additional elements, to cope with the errors.
Since the fault tolerance method has an increased cost due to the duality of the system, many conventional systems adopt the fault prevention method to increase the reliability of the system. However, it is almost impossible to realize a perfect system that takes all of the expected errors into account in advance. Further, since the fault tolerance method, which deals with an error that has already been generated in the system, and does not require a perfect system, the system may be realized with inexpensive standard elements. However, the fault prevention method uses the high reliability expensive elements in order to prevent for sure the error generation, so that the cost may increase undesirably. Namely, fault prevention has a limitation of preventing the errors and has a high cost. Thus, fault tolerance has recently been used for improving the reliability of the system.
The fault-tolerant system refers to a system that can sustain the normal operation without external assistance, even when the system has an error during operation. In order to realize such a fault-tolerant system, the main elements of the system are dualized. Thus, if an element has an error, a corresponding element will take the place of the fault element to sustain the normal operation. Further, if applied to a large-sized exchange system, the fault tolerance system has a very high cost. Accordingly, it is preferable to apply the fault tolerance for a private branch exchange (PBX). Moreover, although the system can be dualized into several modules, it is preferable to dualize the system into two modules, i.e., an operating section and a waiting section, taking the cost into consideration.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited technique for realizing a fault tolerant ISDN PBX as in the present invention: U.S. Pat. No. 4,831,617 to Iwasaki, entitled Data Communication System Having Means For Switching Between Main And Stand-By Apparatuses, U.S. Pat. No. 5,596,569 to Madonna et al., entitled Telecommunications Switch With Inproved Redundancy, U.S. Pat. No. 4,912,698 to Bitzinger et al., entitled Multi-Processor Cential Control Unit Of A Telephone Exchange System And Its Operation, U.S. Pat. No. 4,860,333 to Bitzinger et al., entitled Error Protected Central Control Unit Of A Switching System And Method Of Operation Of Its Memory Configuration, U.S. Pat. No. 4,633,039 to Holden, entitled Master-Slave Microprocessor Control Circuit, U.S. Pat. No. 5,469,503 to Butensky et al., entitled Method For Resynchronizing Secondary Database And Primary Database With Preservation Of Functionality Of An Automatic Call Distribution System, U.S. Pat. No. 5,619,252 to Nakano, entitled Video Telephone System And Method For Transmitting And Receiving Signals When There Is A Failure In The System, U.S. Pat. No. 5,546,452 to Andrews et al., entitled Communications System Using A Central Controller To Control At Least One Network And Agent System, U.S. Pat. No. 5,515,429 to Kawada et al., entitled Backup Apparatus Against Line Defect And Backup Method Against Line Defect, U.S. Pat. No. 5,479,396 to Kusano, entitled Redundant System Having Signal Path Operation Function, and U.S. Pat. No. 5,509,065 to Fitzgerald, entitled Dual Span Monitoring System For Maintenace Shelf Control.